Electrostatic particle charger, electrostatic separation system, and related methods

ABSTRACT

In one aspect of the invention, a charger for use in a system for separating particles from a fluid flow is disclosed. In one embodiment, the charger comprises a body including an inlet for receiving the particles, a chamber in which the particles are received, and an outlet for discharging the particles. A rotor having a generally non-permeable surface is positioned in the chamber and rotated for contacting and charging the particles. In another aspect of the invention, grid electrodes with elongated fingers are proposed for use in a novel separation system. Related methods of charging and separating particles are also disclosed.

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/477,443, the disclosure of which is incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to the material separation art and, moreparticularly, to an improved particle charger or charging device, animproved separator, and related methods for electrostatically separatingtwo species of particles from a particle mixture.

BACKGROUND OF THE INVENTION

“Dry” triboelectrostatic separation is widely used as an effectivetechnique for separating different particulate solid components(“particles”) from a physical mixture entrained or carried in a drivingfluid, such as air. Typical applications include the beneficiation ofminerals, purification of foods, the recovery of valuable componentsfrom waste, and the sizing of particles in a particle mixture. Thistechnology has gained widespread acceptance as providing a low cost,environmentally friendly technique, since it requires no chemicals orwater and thus eliminates costly downstream de-watering and slimedisposal applications required in wet separation processes.

Typically, electrostatic separation relies on the surface physicalproperties of the different particles and controlled flow conditions toeffect beneficiation in an efficient and effective manner. Specifically,when two species of particles with different work functions contact oneanother, a charge transfer between the contact area results, such thatone species may carry a positive charge and the other a negative charge(known as “contact charging”). This differential charge may also beachieved by “friction charging,” which results when the particles areforced to slide along or rub against a solid surface. The combinedeffects of these charges are together known as “triboelectrostaticcharging” or “tribocharging” for short, and are together considered toplay a key role in achieving particle separation.

FIG. 1 schematically illustrates a typical prior art triboelectrostaticseparator S. The particles P in the mixture are fed into the separator Sfrom a bin B, and are charged to a bipolar state in a metal tube T,mainly by friction charging. The particles P then pass through anelectric field F, such that the species of particles having a particularcharge is drawn from the mixture toward a corresponding electrode E₁,E₂. However, as a result of the inefficient charging resulting from thefact that not all particles make contact with the sidewalls of the tubeT, weakly charged or charge-neutral particles may not be attracted andconsequently simply pass through the separator S unaffected by theelectric field F. While these “middling” particles (not shown in FIG. 1)may be separated during a second pass, this obviously decreases theefficiency of the separation operation. Increasing the feed rate of theparticles P may allow for more passes in a shorter period, but aconcomitant decrease in the separation efficiency per pass resultsbecause of the shorter residence time of the particles in the electricfield F.

Accordingly, while the typical prior art separator S is effective forseparating two particle species from a particle mixture, it should beappreciated that further improvements in separation effectiveness andoperational efficiency are still possible. More specifically, a needexists for devices and methods that enhance the charging on theparticles as well as the downstream separation to improve efficiency andpotentially reduce the need for the number of passes required.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, an apparatus forintended use in charging particles in a system for separating particlesfrom a fluid flow is disclosed. The apparatus comprises: (1) a chamberincluding an inlet for receiving the particles and an outlet fordischarging the particles; and (2) a rotor rotatably mounted in thechamber. The rotor has a generally non-permeable outer surface forcontacting and assisting in charging the particles.

In one particular embodiment, the rotor is circular, polygonal, orgear-shaped in cross-section, and the chamber is generally cylindrical.Preferably, the outlet of the chamber is positioned below and generallyopposite the inlet. A partition may also project into the chamberadjacent the rotor. Preferably, the partition is adjustable to vary thedistance between an end of the partition and the rotor. Additionally, amotor is provided for rotating the rotor. The motor may rotate the rotorat a rotational speed of up to 10,000 revolutions per minute.

In the same or another embodiment, an electric field is provided in thechamber. Preferably, the electric field is created by a variable voltagesource having a first lead connected to the rotor and a second leadconnected to a wall of the chamber. The electric field helps to enhancethe charging of certain types of particles.

In accordance with a second aspect of the invention, an apparatus forintended use in separating particles of a mixture is disclosed. Theapparatus comprises a body including an inlet for receiving theelectrically charged particles to be separated, a separation chamber, afirst electrode for attracting particles having a first selected charge,and a second electrode for attracting particles having a second selectedcharge. The first and second electrodes are grid electrodes having aplurality of elongated fingers extending along the separation chamberspaced apart from the body. A flow straightener positioned in oradjacent to the inlet receives and straightens a co-flow of fluid, suchas a gas, passing over and between the fingers of the grid electrodesfor carrying or sweeping away the particles.

In one embodiment of the separation apparatus, a variable voltage sourceapplies a positive voltage potential to the first electrode and anegative voltage potential to the second electrode. Preferably, thefingers on each electrode are connected to a common header.

In accordance with a third aspect of the invention, a method ofseparating particles from a particle mixture is disclosed. The methodcomprises actuating a rotor to create a differential charge on the twoor more constituent species of particles in the mixture and separatingthe differentially charged particles into the two or more constituentspecies at a location downstream of the chamber. Preferably, theactuating step is accomplished by rotating the rotor at a speed of atleast 1,200 revolutions per minute.

In accordance with a fourth aspect of the invention, a method forseparating electrostatically charged particles from a mixture isdisclosed. The method comprises introducing the charged particles to aseparation chamber including a positive grid electrode for attractingnegatively charged particles and a negative grid electrode forattracting positively charged particles; and sweeping away correspondingparticles from the grid electrodes using a straightened co-flow of afluid, such as a gas. The step of actuating a rotor in a mixing chamberupstream of the separation chamber to enhance the charge on theparticles in the mixture may also be performed.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 schematically illustrates a prior art separation system;

FIG. 2 is a partially cross-sectional, perspective view of oneembodiment of the charger forming one aspect of the invention;

FIG. 3 is a graph illustrating the enhanced particle charging achievedwhen an electric field is applied to the charger;

FIGS. 4 a-4 c show exemplary shapes of rotors;

FIG. 5 is a partially cross-sectional, perspective view of oneembodiment of the separator forming another aspect of the invention;

FIGS. 6 a and 6 b are schematic side views of the separator of FIG. 5 inoperation; and

FIG. 7 illustrates an experimental set-up using the charger of FIG. 2and the separator of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the partially schematic, cross-sectional side view ofFIG. 2, and in accordance with a first aspect of the invention, animproved particle charging device or charger 10 is disclosed. Thecharger 10 includes a generally rectangular, elongated inlet 12 forreceiving a feed stream FS, which may include a mixture comprised of atleast two species of particles to be differentially charged. Particlesin the feedstream (which includes at least a small amount of a drivingfluid, such as air) passing through the distributor are introduced tothe inlet 12 and enter a charging chamber 14 forming part of the charger10.

The charging chamber 14 is formed between the inner surface of an outerwall 16 and the outer surface of a charging roller or rotor 18 mountedto rotate about an axis of rotation X, and thus creates an annular spacefor receiving the particle mixture. The roller or rotor 18 is providedwith a generally continuous, non-permeable outer surface for contactingand frictionally charging the particles in the mixture (which typicallyhave a size ranging from 2-3 millimeters or less).

An outlet 20 is defined in the outer wall 16 of the charger 10 generallyopposite the inlet 12. The outlet 20 may be in direct or indirectcommunication with a downstream separator or like device for effectingfurther processing of the particle mixture. A plastic adaptor 22 mayalso be connected to the outlet 20 for receiving and containing theparticle mixture as it transitions to the downstream separator S. Toincrease the throughput without compromising efficiency, the charger 10and all components forming it are elongated in a direction aligned withthe axis of rotation of the rotor 18 (which is shown as being hollow andhaving a center support shaft (not numbered) in operative engagement atone end with a motor M).

In one possible mode of operation, the rotor 18 is rotated at a selectedrotational speed (e.g., up to 10,000 rpm, and more preferably between1,200 and 8,000 rpm) by the motor M (which may be a variable speedelectric motor). Particles encountering the rotor 18 upon passingthrough the inlet 12 become agitated and charged by both friction andcontact charging. More particularly, the dynamic agitation of themixture created by the rotation of the rotor 18 increases the incidenceof both: (1) particle-particle contact, thus creating contact charging;and (2) particle-wall contact (either the outer wall 16 or with thesurface of the rotor 18), thus creating friction charging. In otherwords, the particles in the mixture will have multiple areas of contact,both with the rotor 18 and the other particles, due to the fast rotationand agitation of the particles created thereby. As a result of usingthis “rotary charger,” a much higher charge density on the surface ofthe particles results, and the incidence of weakly or neutrally chargedparticles passing through the outlet 20 is reduced.

When the particles passing through the charger 10 are fed to adownstream separator S, separation efficiency is increased (possibly byas much as 40%) and the need for multiple passes to effect separationmay be eliminated. The active charging provided by the charger 10 alsoallows for a much higher throughput without reducing the separationefficiency, as compared to the passive charging afforded by thetube-type of arrangement shown in FIG. 1. The charger 10 also helps toensure that all particles are charged, not just a mono-layer ofparticles at the surface of the mixture (as is the case of a coronacharger).

The charger 10 may also operate in a continuous fashion such thatparticles fed through the inlet are constantly being charged anddischarged through the outlet for downstream separation. However, theprovision of a closure or door adjacent the outlet 20 is a possibility,including in the case where the operation of the charger is separatefrom the downstream operation. In other words, the charging may becompleted apart from the separation, the two may occur simultaneously onthe same batch of the particle mixture, or the two may occursimultaneously on two different batches of the particle mixture.

FIG. 2 also illustrates that a partition 24 may also be provided forselective insertion into the chamber 14 to perform the dual function ofpreventing the particle mixture from prematurely entering the outlet 20in one direction and guiding the particle mixture to the outlet in theother. The partition 24 may pass through an opening in the outer wall16, preferably adjacent to the opening defined by the outlet 20 throughwhich the particle mixture exits the chamber 14, and its inner endextends to a point closely adjacent to the outer surface of the rotor18. This inner end of the partition 24 may have an upper face matchingthe contour of the rotor (e.g., an arcuate face, in the case where therotor is cylindrical)). The partition 24 may be mounted directly to thewall defining the outlet 20 using a fastener (FIG. 2), and mayoptionally be mounted to permit selective adjustment of the inner endtoward or away from the rotor 18.

When the rotor 18 rotates in the clockwise direction as viewed in FIG. 2(note action arrow A), the partition 24 is thus positioned downstream ofthe outlet 20 in the angular sense. In this position, it serves toprevent or block particles from simply falling through the outlet 20without making contact with the surface of the rotor 18 or the insidesurface of the outer wall 16. The partition 24 so positioned alsoprevents lighter particles from becoming permanently suspended in thefluid flow surrounding the rotor 18 during rotation, since it contactsand forces the particles into the outlet 20 and toward the downstreamseparator. As should be appreciated, when the direction of rotation isreversed, the position of the partition 24 relative to the outlet 20 maybe likewise reversed to accomplish the intended blocking and guidingfunctions.

Selective charging may further be enhanced by applying an electric fieldto the charger 10. Specifically, as shown in FIG. 2, the leads of anexternal voltage source 26 are applied to the rotor 18 and the outerwall 16 of the chamber 14 to create an electric field therein. Usingthis externally applied voltage may allow for a certain charge densityand polarity to be achieved on the particles. For example, asgraphically illustrated in FIG. 3, using a typical phosphate and quartzmixture, the phosphate may be charged over a range of 500×10⁻⁶ C/kg(from about positive 250×10⁻⁶ C/kg to negative 250×10⁻⁶ C/kg) while thequartz is always charged negatively when the external applied voltage isin the range of −9 kV to +9 kV. At zero voltage, both the phosphate andquartz are charged negatively. Therefore, separation of phosphate fromquartz is more efficient if an external voltage is applied.

Although a generally cylindrical rotor 18 is shown in FIG. 2, it shouldbe appreciated that other shapes may be used (and that such shapes mayfurther enhance the charging of the particles). An example of a gear orsprocket-wheel type of rotor 18 a with outwardly projecting “teeth” andcorresponding surface indentations is shown in FIG. 4 a. An octagonalrotor 18 b is shown in FIG. 4 b, which thus provides a plurality (eight)of distinct contact surfaces. Other polygonal shapes could also be used(e.g., a hexagon or dodecagon), with or without rounded corners at theintersections of the planar surfaces. FIG. 4 c also shows that acylindrical separator 18 c may be formed as a solid body, as compared tothe hollow body 18 shown in FIG. 2. In both cases, the outer surface ofthe rotor 18 remains non-permeable or continuous.

In accordance with another aspect of the invention, an improvedseparator 100 is also disclosed. The separator 100 includes adistributor 112 defining an inlet for receiving a feedstream of chargedparticles (which as should be appreciated may be delivered from theoutlet 20 of the charger 10 described above or a different device,including the conventional tube T shown in FIG. 1). The particles aredelivered to a body 114 of the separator 100, which is generallyrectangular and elongated. The driving fluid (gas) may be supplied by adriving fluid source, such as a forced draft fan (not shown), positionedupstream of the distributor 112. The fluid or gas is preferably ambientair, but other gases such as nitrogen, helium, argon, carbon dioxide, orcombustion flue gas can be used at temperatures between approximately25° C. to 300° C.

In typical separators using plate-type electrodes (see FIG. 1), thedeposition of charged particles may reduce the separation efficiency andrequire deleterious periodic shutdowns for cleaning. In an effort toreduce or eliminate the need for such shutdowns, a pair of grid or grillelectrodes 116, 118 are positioned in the body 114, spaced from thesidewalls thereof. Each grid electrode 116, 118 is comprised of aplurality of elongated, generally parallel fingers 120 that extend intoand define a separation chamber 122 within the body 114. The fingers 120each emanate from a common header 124 and a similar footer (not shown)may also be provided to enhance the rigidity and stability of theelectrodes 116, 118. The elongated, spaced nature of the fingers 120eliminates or substantially reduces the amount of particles deposited onthe electrodes 116, 118, which obviates the above-mentioned problems.

Each electrode 116, 118 is connected to the lead of a variable voltagesource 126 (such as along the header 124) to create an electric field inthe chamber 122 for separating the particles having a selected charge. Aco-flow of gas devoid of particles may also be introduced from aseparate source (not shown) for sweeping away the particles drawntowards the electrodes 116, 118. Preferably, flow straighteners 128 areprovided to reduce the turbulence and form a smooth co-flow of gasgenerally parallel to the feedstream FS upon entering the separationchamber 122. The flow straighteners 128 may be in the form of tubeshaving aspect ratios, i.e., the ratio of length to diameter, of greaterthan 20:1, but other types of straighteners (such as vanes) may also beused.

FIGS. 6 a and 6 b schematically demonstrate a comparison between the useof plate electrodes P (FIG. 6 a) and the grill or grid electrodes 116,118 (FIG. 6 b). In FIG. 6 a, the deposit D of particles on the plateelectrodes P is shown. Since the flow of both the feedstream FS ofcharged particles and the co-flow CF devoid of particles passes onlyover the opposed faces of the plate electrodes P, the particles drawnfrom the mixture accumulate and form the deposits D. However, in thecase of the grid electrodes 116, 118 (which are spaced from at least twoadjacent sidewalls of the body 114, and preferably all four), the flowsessentially surround the fingers, moving both over and between them.This helps to prevent the particles from accumulating and forming theundesirable deposits that hamper efficient operation.

FIG. 7 shows an experimental set-up built and used to demonstrate theeffectiveness of the charger 10 and separator 100 disclosed herein whenused in combination. As is known in the art, the system may include asplitter 140 downstream of the separator for dividing the flow intostreams including the substantially separate and pure species ofparticles and cyclones 150 or other filtering devices for removing theparticles from the streams once separated. Collection bins 160 may alsobe provided for collecting the first and second species of particles, aswell as any “middlings” that result.

Experiments were conducted using the exemplary system 100 shown in FIG.7 in an effort to demonstrate the efficacy of the charger 10 Bothone-stage and two-stage separation was employed. Using this set-up, thefollowing sets of data were obtained using both one-stage and two-stageseparation:

EXAMPLE 1 Fly Ash Separation Result

TABLE 1 One-stage fly ash separation Ash Middling Tailing # LOI, % YieldLOI, % Yield LOI, % Yield #1 0.75 44.57 1.92 43.30 4.31 12.13 #2 0.9459.92 7.49 25.62 39.19 14.46 #3 1.19 33.87 2.82 38.09 15.98 28.04 #41.21 42.08 5.37 45.82 28.47 12.10 #5 3.64 43.41 14.93 39.22 41.44 17.37

EXAMPLE 2 Fly Ash Separation Result

TABLE 2 Two-stage fly ash separation Product ΣProduct Product ΣProductΣAsh Ash, % Ash, % Yield, % Yield, % Recovery, % 0.23 0.23 36.01 36.0138.85 1.04 0.48 15.58 51.59 55.52 3.58 0.72 4.48 56.06 60.18 6.04 1.316.90 62.96 67.19 7.12 1.67 4.20 67.16 71.41 9.64 3.11 14.85 82.01 85.9213.03 3.73 5.45 87.45 91.04 27.13 4.50 2.98 90.43 93.38 30.84 5.94 5.2495.67 97.30 42.38 7.52 4.34 100.00 100.00

EXAMPLE 3 Coal Cleaning Result

TABLE 3 Coal cleaning Product ΣProduct Product ΣProduct ΣCombustibleΣAsh Ash, Ash, Yield, Yield, Recovery, Rejection, % % % % % % 3.44 3.4442.40 42.40 49.56 91.61 7.82 4.84 19.94 62.34 71.81 82.64 26.92 9.0714.75 77.10 84.86 59.78 37.89 13.02 12.24 89.34 94.06 33.09 53.96 17.3810.66 100.00 100.00 0.00

Table 3 shows the results of coal cleaning obtained by a two-stageclosed circuit test. The raw coal ash content is about 17%. For theproduct with 9.07% ash, an 84.86% of combustible recovery can beachieved with an ash rejection of 59.78%.

EXAMPLE 4 Ground Calcium Carbonate Separation Result

TABLE 4 Separation results on ground calcium carbonate (GCC) InsolubleΣInsoluble Yield ΣYield ΣRecovery % % % % % One-Stage 0.50 0.50 40.7040.70 41.99 3.00 1.88 50.10 90.79 92.39 20.30 3.58 9.21 100.00 100.00100.00 Two-Stage 0.10 0.10 14.61 14.61 15.11 0.50 0.33 19.61 34.22 35.310.50 0.39 19.20 53.42 55.09 1.80 0.50 4.34 57.76 59.51 2.50 1.16 28.3186.07 88.09 6.40 1.35 3.37 89.43 91.35 12.50 1.83 3.99 93.42 94.96 16.602.54 4.71 98.13 99.03 49.90 3.42 1.87 100.00 100.00 100.00

As shown in Table 4, efficient removal of silica from the ground calciumcarbonate (GCC) was achieved with the triboelectrostatic separationtechnology. A two-stage separation produced better separation resultsthan the one-stage separation. Based on the two-stage separation,approximately 34% of calcium carbonate can be recovered for a productwith 0.3% insol; a 57% yield of calcium carbonate is expected for aproduct with 0.5% insoluble.

EXAMPLE 5 Phosphate Separation Result

TABLE 5 Two-stage separation on phosphate flotation feed ΣP₂O₅ P₂O₅ %ΣP₂O₅ Yield % ΣYield Recovery % 36.64 36.64 5.15 5.15 32.35 17.32 23.4811.00 16.14 65.03 14.42 21.48 4.55 20.70 76.29 12.96 19.99 4.40 25.0986.06 3.21 13.82 14.58 39.67 94.08 1.99 12.26 6.03 45.70 96.14 1.0110.47 8.64 54.34 97.64 0.36 7.23 25.68 80.02 99.23 0.22 5.83 19.98100.00 100.00

Two-stage separation was conducted on a phosphate sample (Table 5),which is the flotation feed. Two fractions containing less than 0.5%P₂O₅ with 45% yield exist. A concentrate with 36.64% P₂O₅ can beproduced with 32.35% P₂O₅ recovery.

The foregoing descriptions of various embodiments of the invention areprovided for purposes of illustration, and are not intended to beexhaustive or limiting. Modifications or variations are also possible inlight of the above teachings. The embodiments described above werechosen to provide the best application to thereby enable one of ordinaryskill in the art to utilize the disclosed inventions in variousembodiments and with various modifications as are suited to theparticular use contemplated. All such modifications and variations arewithin the scope of the invention as determined by the appended claimswhen interpreted in accordance with the breadth to which they arefairly, legally and equitably entitled.

1. An apparatus for intended use in charging particles in a system forseparating particles from a fluid flow, comprising: a chamber includingan inlet for receiving the particles and an outlet for discharging theparticles; and a rotor rotatably mounted in the chamber, the rotorhaving a generally non-permeable outer surface for contacting andassisting in charging the particles.
 2. The apparatus according to claim1, wherein the rotor is circular, polygonal, or gear-shaped incross-section.
 3. The apparatus according to claim 1, wherein thechamber is generally cylindrical.
 4. The apparatus according to claim 1,wherein the outlet is positioned below and generally opposite the inlet.5. The apparatus according to claim 1, further including a partitionprojecting into the chamber adjacent the rotor.
 6. The apparatusaccording to claim 5, wherein the partition is adjustable to vary thedistance between an end of the partition and the rotor.
 7. The apparatusaccording to claim 1, further including a motor for rotating the rotor.8. The apparatus according to claim 1, wherein the rotor rotates at arotational speed of between about 1,200 and 10,000 revolutions perminute.
 9. The apparatus according to claim 1, further including anelectric field in the chamber.
 10. The apparatus according to claims 9,wherein the electric field is created by a variable voltage sourcehaving a first lead connected to the rotor and a second lead connectedto a wall of the chamber.
 11. A particle separation system including theapparatus of claim
 1. 12. An apparatus for intended use in separatingparticles of a mixture, comprising: a body including an inlet forreceiving the electrically charged particles to be separated, aseparation chamber, a first electrode for attracting particles having afirst selected charge, and a second electrode for attracting particleshaving a second selected charge; wherein the first and second electrodesare grid electrodes having a plurality of elongated fingers extendingalong the separation chamber spaced apart from the body; and a flowstraightener positioned in or adjacent to the inlet for receiving andstraightening a co-flow of fluid passing over and between the fingers ofthe grid electrodes.
 13. The apparatus according to claim 12, furtherincluding a variable voltage source for applying a positive voltagepotential to the first electrode and a negative voltage potential to thesecond electrode.
 14. The apparatus according to claim 12, wherein thefingers on each electrode are connected to a common header. 15.(canceled)
 16. A method of charging particles using the apparatus ofclaim
 1. 17. A method of separating particles using the apparatus ofclaim
 12. 18. A method of separating particles from a particle mixture,comprising: actuating a rotor to create a differential charge on the twoor more constituent species of particles in the mixture; and separatingthe differentially charged particles into the two or more constituentspecies at a location downstream of the chamber.
 19. The method ofseparating particles according to claim 18, wherein the actuating stepcomprises rotating the rotor.
 20. A method for separatingelectrostatically charged particles from a mixture, comprising:introducing the charged particles to a separation chamber including apositive grid electrode for attracting negatively charged particles anda negative grid electrode for attracting positively charged particles;and sweeping away corresponding particles from the grid electrodes usinga straightened co-flow of fluid.
 21. The method according to claim 20,further including the step of actuating a rotor in a mixing chamberupstream of the separation chamber to enhance the charge on theparticles in the mixture.